This invention relates to an antenna unit including a patch antenna and, in particular, to an antenna unit including a patch antenna which is used as an antenna for use in a digital radio receiver for receiving an electric wave from an artificial satellite (that may be called a “satellite wave”) or an electric wave on the ground (that may be called a “terrestrial wave”) to listen in a digital radio broadcasting.
In recent years, a digital radio receiver, which receives the satellite wave or the terrastrial wave to listen the digital radio use in the United States of America. The digital radio receiver is mounted on a mobile station such as an automobile and can receive an electric wave having a frequency of about 2.3 gigahertz (GHz) to listen in a radio broadcasting. That is, the digital receiver is a radio receiver which can listen in a mobile broadcasting. Inasmuch as the received wave has the frequency of about 2.3 GHz, a reception wavelength (resonance frequency) l thereof is equal to about 128.3 mm. In addition, the terrestrial wave is an electric wave in which a signal where the satellite wave is received in an earth station is frequency shifted a little.
Inasmuch as the electric wave having the frequency of about 2.3 GHz is used in the digital radio broadcasting in the manner which is described above, it is necessary to set up an antenna outside the automobile. Accordingly, the antenna must be attached to a roof of the automobile in a case where the digital radio receiver is mounted in the automobile.
It is necessary for the antennas of the type described to have a wide directivity. This is because it is necessary for the digital radio receiver to receive, from the artificial satellite, the satellite wave having a relatively low angle of elevation which laid in a range of 20 degrees and 60 degrees. In addition, the terrestrial wave has an angle of elevation which is substantially equal to zero degree.
In general, it is understood that antennas of planer-type (plane-type) such as patch antennas are unsuited for the antenna for use in the digital radio receiver because the antennas of planer-type (plane-type) has a narrow directivity. However, even a patch antenna, it is known that it is possible for the patch antenna to widen a directivity of an antenna by using a ground (earth) plate having a wide area (see, e.g. JP 2003-163521 A).
Particularly, in a case where the digital radio receiver is mounted in the automobile, its antenna is attached to the roof of the automobile in the manner which is described above. In this event, inasmuch as the roof of the automobile serves as the ground plate in itself, it is confirmed that it is possible for the patch antenna to sufficiently use the antenna for the digital radio receiver.
On the other hand, there is a demand to listen in the digital radio broadcasting not only in the inside of the automobile but also, for example, from a portable audio apparatus such as a compact disc (CD) radio-cassette recorder. In order to reply the demand, it is proposed to mount the patch antenna in a lid which is arranged on a top surface of a casing in the portable audio apparatus and which is openable for taking an optical disk into and out from the portable audio apparatus. In this event, it is necessary to provide with the ground plate (earth plate) under the patch antenna.
In the manner which is described above, it is necessary for the patch antenna to use the ground plate having a large area in order to gain the directivity of the antenna in the low angle of elevation. However, it is difficult for the above-mentioned portable audio apparatus to be provide with the ground plate (earth plate) having a wide area. Accordingly, an antenna unit having the directivity of the antenna which is not degraded in the low angle of elevation is desired in a case of using the ground plate (earth plate) having a narrow area.
In addition, a planar antenna is known. The planar antenna comprises a circuit substrate having an upper surface and a bottom surface, a patch antenna mounted on the upper surface of the circuit substrate, circuit elements such as a low noise amplifier (LNA) and so on mounted on the bottom surface of the circuit substrate, a shield cover, attached to the bottom surface, for shielding the circuit elements (see, e.g. JP 2002-26649).
In addition, a compact plane patch antenna for use in a global positioning system (GPS) or the like (see, e.g. JP 07-094934 A). According to JP 07-094934 A, the compact plane patch antenna has high infrequency temperature characteristics and high reliability by using magnesium titanate ceramic having comparatively high dielectric constant as a main material for a dielectric material and adding the proper quantity of lithium niobate, alumina, manganese oxide, etc., individually or their combination at ions to the main material to mold the antenna.